WhatsApp vulnerability: Bug or Backdoor?

In April last year I reported on a vulnerability in WhatsApp. It now gained public attention as it has not been fixed yet. One question now is: Is it a bug or a backdoor? In other words: Is this flaw put deliberately into the WhatsApp messenger to allow them or the government to look at targeted messages? Or is the flaw introduced through a simple programmer error? Or is it even a useful feature?
If someone would demand WhatsApp to implement a backdoor, you might expect them to implement something more obvious. Like responding with the history of all conversations when triggered to so do with a certain secret message. Furthermore, this flaw can be explained as a programming bug. Just a missed "if" statement for one of the corner cases. It is a type of flaw that is not necessarily introduced by malice, just like many other critical vulnerabilities in important products that are reported daily.
But Facebook showed no interest in fixing the flaw since I reported it to them in April 2016. So maybe it was a bug first, but when discovered it got started being used as a backdoor?
WhatsApp has stated recently that this is not a bug, it is a feature! Because now senders don't have to press an extra "OK" button in the rare case they sent a message, the receiver is offline and has a new phone when coming back online. That's not a very good argument! And if "Privacy and Security is in [WhatsApp's] DNA", they should have fixed the flaw immediately after I reported it in April 2016.